1. Field of the Invention
The present invention relates generally to refrigeration systems and, more particularly, to a liquid pump for a refrigeration system.
2. Description of the Related Art
A typical refrigeration system utilizes a control valve, commonly called an "Expansion valve", to meter the flow of liquid refrigerant to an evaporator of the refrigeration system. Sometimes, conditions exist which may cause vapor (commonly called flashgas) to form in the refrigerant feeding the expansion valve. These conditions may arise from either (1) a drop in pressure, due to piping pressure drop or rise in elevation, or (2) an increase in refrigerant temperature, due to heat transfer from ambient surroundings. Since the expansion valve is specifically designed to handle liquid refrigerant only, the presence of flashgas may interfere with the proper operation of the expansion valve.
A liquid pump can be used to slightly elevate the pressure of the refrigerant which feeds the expansion valve, and subsequently eliminate the formation of flashgas. Conventional centrifugal pumps are generally not suitable for this application. These conventional pumps consist of an impeller which is located in a housing and powered by a drive shaft which penetrates the housing. This arrangement requires a seal between the rotating shaft and the stationary housing, and thus provides a potential leakage of refrigerant from the refrigeration system.
Currently, there are two liquid pumps which eliminate the above seal. One liquid pump uses a magnetic clutch to drive a pump impeller with a conventional motor. The other pump incorporates a special motor which uses a thin metallic membrane to isolate the motor windings from the refrigerant. The operation of both pumps causes a magnetic field to rotate through a stationary metallic barrier, resulting in an energy loss due to eddy currents in the barrier. Also, both pumps have a high manufacturing cost, in comparison, to a conventional pump.